The present invention is directed to membrane introduction devices coupled to a mass spectrometer to introduce volatile molecules in solution into the ion source. Most of that work has coupled a membrane interface to an electron ionization (EI) or chemical ionization (CI) source. As a result, the volatile molecules that permeate the membrane go under a pervaporization process on the other side of the membrane where they are desorbed from the membrane surface and can then be ionized in the gas phase. For more detailed information please see Mark Bier's Ph.D. Thesis, Purdue University 1988 (incorporated herein by reference).
Membrane introduction mass spectrometry (MIMS) is a sensitive, selective technique for the analysis for many small organic compounds in water in real-time. In this technique, small molecules are selectively concentrated by adsorption and separated from the water by preferential permeation through a membrane.
First used by Hoch and Kok (see A Mass Spectrometer Inlet System for Sampling Gases Dissolved in Liquid Phases. Arch. Biochem. Biophys. 1963, 101(1), 160-170) as a method for monitoring the production and consumption of gases during photosynthesis, MIMS has since been developed as a versatile analytical method that is applicable to a wide variety of analyses. For example, MIMS has been used to monitor the ethanol content of a fermentation broth, biological reactions, the destruction of environmental contaminants, the chlorination of organic amines in water, kinetic and mechanistic aspects of chlorination of organic compounds in water, organometallic compounds in water, and volatile and semi-volatile organic compounds (VOC and SVOC) in a variety of matrices. Most VOCs have been analyzed by MIMS in air or in aqueous solution, but they have also been analyzed in soil and human breath.
There are many methods available to analyze VOC and SVOC in water such as headspace analysis and purge-and-trap coupled to gas chromatography (GC) or gas-chromatography-mass spectrometry (GC-MS). Headspace analysis requires little time for sample preparation, but generally can be used only for contaminants at high concentrations. Purge-and-trap analysis is a recommended concentrating method by the Environmental Protection Agency (EPA), but it often has problems of sample carryover and contamination and it is also an expensive method that requires a bulky apparatus. Liquid-liquid extraction, and solid-phase micro extraction are commonly used for analyzing SVOCs. Liquid-liquid extraction methods generally require large quantities of solvents which makes them more expensive and less environmentally friendly. Solid-phase micro extraction, while fast compared to some other methods mentioned here, can require tens of minutes of off-line sample preparation prior to analysis and requires an individual solid phase concentrating cartridge per sample.
Early MIMS systems were plagued with problems of irreproducibility, membrane memory effects, long response times and high detection limits (see Johnson, R. C.; Cooks, R. G.; Allen, T. M.; Cisper, M. E.; Hemberger, P. H. Membrane Introduction Mass Spectrometry: Trends and Applications. Mass Spectrometry Reviews. 2000, 19(1), 1-37). To effectively analyze low level environmental contaminants, design improvements have been made to these devices. For example, a direct insertion membrane probe was built to place a capillary membrane directly inside the ionization source of a mass spectrometer located millimeters from the electron ionizing beam (see Bier, M. E.; Cooks, R. G. Membrane Interface for Selective Introduction of Volatile Compounds Directly into the Ionization Chamber for a Mass Spectrometer. Anal. Chem. 1987, 59(4), 597-601). The close proximity of the membrane to the ionization region allowed for rapid reproducible analysis of VOCs in water at low detection limits with minimal memory effects because the analyte was instantaneously ionized with little mixing rather than flowing inside a transfer line and the membrane was heated to increase permeation rates. In 1991, Silvon et al. developed a helium-purged hollow fiber membrane interface that allowed for detection at the sub part-per-billion (ppb) levels in the analysis of VOCs and SVOCs (see Silvon, L. E.; Bauer, M. R.; Ho, J. S.; Budde, W. L. Helium-Purged Hollow Fiber Membrane Mass Spectrometer Interface for Continuous Measurement of Organic Compounds in Water. Anal. Chem. 1991, 63(13), 1335-1340). In Silvon's device, a capillary membrane is placed inside a flow cell with a helium purge running through the membrane to the mass spectrometer while the aqueous sample runs outside the membrane in the opposite direction. The best detection limit has come from Soni et. al. who detected 500 parts per quadrillion (ppq) of toluene with a S/N 11 with data point smoothing (see Soni, M. H.; Baronavski, A. P.; McElvany, S. W. Trace Analysis of Polyaromatic Hydrocarbons in Water Using Multiphoton Ionization-Membrane Introduction Mass Spectrometry. Rapid Commun. Mass Spectrom. 1998, 12, 1635-1638). In this experiment, a stored wave form inverse Fourier transform (SWIFT) signal was applied to the end caps of a 3D quadrupole ion trap. The SWIFT waveform isolates desired analyte ions by ejecting only those contaminant ions at the frequencies in the SWIFT signal. The use of this waveform and other similar waveforms allowed for a significant concentration of the ions of interest.
Finally, U.S. Pat. No. 6,360,588 to Ross et al. discloses an efficient and accurate method and apparatus for analysis of materials passing through a membrane. A sample is place on one side of a membrane and a carrier fluid from a reservoir flows past the other side of the membrane to carry any sample diffusing through the membrane to be detected. The disclosed method can allow for the accurate, precise, and specific real time measurements of compounds crossing a membrane.